The change in frequency of a wave (such as sound) for an observer moving relative to the source of the wave/sound is referred to as the Doppler effect or Doppler shift. Leveraging the Doppler effect, laser Doppler vibrometers can be used to make non-contact vibration measurements of a surface. In a laser Doppler vibrometer, a laser beam is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the reflected laser beam frequency due to the motion of the surface.
In the context of ultrasound, the Doppler ultrasound technique relies on the use of pulsed emissions. In pulsed Doppler ultrasound, instead of emitting continuous ultrasonic waves, an emitter periodically sends a short ultrasonic burst. A receiver can continuously receive echoes from targets that may be present in the path of the ultrasonic beam. By sampling the incoming echoes at the same time relative to the emission of the bursts, the shift of positions of scatters are measured. This has various applications, e.g., for measuring the velocity of fluids such as blood. For example, an ultrasound transducer can be used to transmit ultrasound into tissue. If the ultrasound passes through a vessel with blood, the ultrasound will interact with the blood cells and a part of the ultrasound energy will be scattered back towards the transducer, where it can be detected.
Doppler ultrasound can also be accomplished using continuous ultrasonic waves at known frequencies. When incident on a moving body, the reflected waves are shifted by a frequency proportional to the velocity of the moving body. By comparing the frequencies of the reflected waves to the original ultrasonic signal, a detailed description of the motion of the moving body can be constructed. For a single frequency of emitted ultrasound, this comparison is done through frequency demodulation using the emitted frequency as the zero point. This is superior to the pulsed ultrasound technique as it provides a continuous measurement rather than a pulsed one. In this way, bandwidth is greatly increased.